Selective hydrogenation of hydrocarbons



United States Patent 3,309,307 SELECTIVE HYDROGENATION 0F HYDROCARBONSHoward S. Bryant, Jr., Beaumont, Tex., assignor to Mobil OilCorporation, a corporation of New York No Drawing. Filed Feb. 13, 1964,Ser. No. 344,544 Claims. (Cl. 208-144) The present invention relates tothe selective, nondestructive hydrogenation of mixtures containinghighly unsaturated hydrocarbons and olefins in which the highlyunsaturated substances are preferentially hydrogenated. Thermallycracked petroleum hydrocarbons are a preferred feedstock for saidselective hydrogenation. In a particular embodiment, it is concernedwith subjecting a pyrolysis gasoline to a mild hydrogenation treatmentof improved selectivity to form a stabilized liquid product suitable foruse either as a motor fuel blending stock or as an intermediate fromwhich aromatic hydrocarbons may be recovered after further processing.

In the production of olefins, especially ethylene and propylene, bysubjecting petroleum fractions, such as naphthas, to severe thermalcracking, usually in the presence of steam, a considerable quantity ofpyrolysis gasoline is produced which is unsuitable for use in motorfuels due to its tendency to form excessive quantities of gum duringstorage. This thermally cracked gasoline contains substantialproportions of both diolefins and mono-olefins as well as some aromaticcompounds (typically about 6 to 50% by volume or more) and perhaps someacetylenic materials. The more reactive diolefins among the diolefinstherein are particularly undesirable by reason of their known tendencyto polymerize and form gums upon prolonged standing. Mono-olefins ingeneral are desirable constituents of motor fuels as they haverelatively high octane ratings, and aromatic hydrocarbons are superiorin this regard.

Conventional hydrogen treatments for stabilizing such hydrocarbonmixtures are not entirely satisfactory because of their lack of adequateselectivity and also the usual relatively high operating temperatures.For example, the hydrogenation may not end with simply partialsaturation of the diolefins to olefins but also frequently saturates themono-olefins completely and even hydrogenates substantial proportions ofthe aromatic hydrocarbons to less valuable naphthenes. Polymerization ofdiolefins with consequent contamination and deactivation of the catalystwith gummy deposits or coke often occurs. Such polymerization may be ofthe thermal type induced by high temperatures, or it may be of acatalytic type inaugurated by the hydrogenation catalyst, as goodhydrogenation catalysts frequently possess substantial polymerizationactivity also. The polymeric deposits are highly undesirable as they notonly reduce the hydrogenation activity of the catalyst, therebyrequiring frequent regeneration but also tend to plug up piping andother equipment.

Selective hydrogenation is also employed in multi-stage hydrogenationreactions as for instance in the preparation of pyrolysis gasoline forthe extraction of its aromatic hydrocarbon content by well-known solventextraction techniques as exemplified by extraction with diethyleneglycol. To prepare a suitable feed for the solvent extraction, it isnecessary to convert the organic sulfur compounds to a readily separablematerial, such as hydrogen sulfide gas, to saturate the unstable gumforming diolefins and also to saturate the mono-olefins without at thesame time converting aromatic hydrocarbons into naphthenes by excessivehydrogenation.

It is not feasible to completely saturate and desulfurize suchfeedstocks in a single operation because the relatively hightemperatures suitable for hydrodesulfurization (typi- 3,369,307 PatentedMar. 14, 1967 cally at least about 450 F.) also promote the formation ofcoke and polymers or gums, and such temperatures may hydrogenatearomatics to naphthenes under certain conditions. Even conducting thehydrogenation reactions in several stages to avoid or minimize theaforesaid difficulties has not been entirely satisfactory by reason ofthe accumulation of polymeric deposits that reduce the activity ofhydrogenation catalysts, thereby requiring frequent regeneration. Inaddition, such deposits also plug up piping and other equipment. Notonly thermal polymerization but also catalytic polymerization must beminimized as many good hydrogenation and desulfurization catalysts alsocatalyze the polymerization of diolefins. While various techniques areknown for at least partially reducing the polymer formation ofhydrocarbons at elevated temperatures, nevertheless polymer formationremains a critical problem in commercial plants for the selectivehydrogenation of charging stocks of the type described.

The present invention involves the selective non-de structivehydrogenation (that is, without substantial cracking or hydrocracking)under mild conditions of hydrocarbons which comprises treating anessentially hydrocarbon mixture containing a conjugated diolefin and anolefin with hydrogen at hydrogenation temperatures below desnlfurizationtemperatures in the presence both of a catalyst containing palladium andalso of a sulfide of the group consisting of carbon disulfide andhydrogen sulfide wherein said sulfide substantially increases theselectivity of said catalyst for hydrogenating said highly unsaturatedhydrocarbons.

Narrower aspects of the invention relate to the presence of carbondisulfide as the preferred agent for improving catalyst selectivity,desirable concentrations of the selected sulfide, the composition of thepreferred palladium composite catalyst, maintaining a substantial liquidphase in the reaction and supplying the selected sulfide either byconversion in the pyrolysis reaction or by at least intermittentaddition to the hydrogenation reaction or both. Other aspects of theinvention will be apparent to those skilled in the art uponconsideration of the detailed disclosure which follows.

Unless otherwise indicated herein, all proportions are expressed interms of weight, all temperatures in degrees Fahrenheit F.), all boilingpoints and ranges are determined according to the ASTM procedure atnormal atmospheric pressure and the term olefins refers to monoolefins.The expression highly unsaturated hydrocarbons may be defined for theinstant purposes as highly reactive hydrocarbons containing aliphaticunsaturation and having a strong tendency toward polymerization atmoderately elevated temperatures such as conjugated diolefins, acetyleneand substituted acetylenes. Unconjugated diolefins are less reactive andgenerally treated as mono-olefins.

In performing the instant process a hydrogenation feedstock with apronounced tendency toward undesired polymerization is subjected to aselective hydrogenation process in which it is hydrogenated mildly inwhat may be described as essentially the liquid phase although thereactor contains a minor proportion of vaporized gasoline along withlarge amounts of hydrogen and usually lower hydrocarbon gases. Thereaction is conducted at a temperature sufiiciently low to avoid orminimize both thermal and catalytic polymerization while hydrogenating asubstantial proportion (usually all or a predominant proportion) of theconjugated diolefins, including all of the more reactive ones, to formeither olefins or paraffins. In addition, the degree of saturation ofmono-olefins in the process is desirably kept as low as may be feasible.

Typically the liquid feed has substantial contents of diolefins andolefins as evidenced by diene numbers of about 10 to 22, which measurethe proportion of conjugated diolefins as determined by the maleicanhydride condensation method, and bromine numbers of about 15 to 30,which represent the total content of unsaturated aliphatic hydrocarbons.Feeds with diene and bromine numbers as high as about 40 and about 75,respectively, may also be processed according to the present invention.A feed containing 6 to 20% of aromatic hydrocarbons is often employedbut higher proportions of these compounds are even more desirable foruse in motor fuel components and especially for feedstocks for theproduction of aromatic hydrocarbons.

Feedstocks of the nature described are unstable as they tend to formpolymeric gums readily. It has been found desirable to keep the periodof storing them as brief as possible in order to minimize theintroduction of gum in liquids involved in the present process and intothe equipment. In addition, it is recommended that the liquid chargestock be free of dissolved oxygen and be stored in the substantialabsence of oxygen or air, for example, under a blanket of an inert gassuch as nitrogen, methane, etc. These precautions prolong the activityof the catalyst.

The pyrolysis reaction employed in one embodiment of the instantinvention is carried out in conventional equipment under noncatalyticand relatively severe thermal cracking conditions for petroleum stocks,as exemplified by temperatures in the range of about 1250 to 1600 F.,

pressures of to 40 pounds per square inch gage (p.s.i.g.) and reactiontimes of about 0.2 to 4.0 seconds. A wide variety of pyrolysis feeds maybe utilized including gas oils, naphthas, middle distillates, pentanesand light, normally gaseous hydrocarbons such as ethane, propane andbutanes. Thsee may be of varying degrees of purity. A substantialproportion of the organic sulfur compounds therein are converted bypyrolysis into carbon disulfide which may be recovered by condensationof the normally liquid fraction of the pyrolysis product or byfractional distillation thereof to produce a cut with an initial boilingpoint below 115 F. Excessive concentrations of carbon disulfide in thepyrolysis product may be reduced to the desired extent by scrubbing withaqueous caustic soda with the rate of introducing caustic soda solutionadjusted to produce the desired reduction of organic sulfur, includingcarbon disulfide. Accordingly, it is possible to obtain a pyrolysisproduct containing a substantial portion or all of the sulfide agentrequired in the hydrogenation step by selection of pyrolysis feedstocksor by blending high and low sulfur content stocks to make up thepyrolysis feed. It is generally not as convenient to attempt to retainany hydrogen sulfide formed in the pyrolysis reaction in the liquidcharge to the hydrogenation reaction, so this gas is usually withdrawnalong with other gaseous products of the pyrolysis reaction.

While pyrolysis gasoline is the prefrered charge to the hydrogenationreaction and the material to be hydrogenated is preferably maintainedchiefly in the liquid phase during hydrogenation, other materials mayalso be selectively hydrogenated. For example, propylene streamscontaining small percentages of methy acetylene and propadiene may besubjected to similar selective hydrogenation in the vapor phase toconvert those two contaminants without saturating the propylene to anunacceptable level. Also undesired acetylene in an ethylene stream maybe converted by selective hydrogenation.

When charging a pyrolysis gasoline as the hydrogenation feed, a gasolineboiling somewhere within the range between about 60 and 400 F. isusually desirable, and preferably one boiling between about 90 and 240F. In general, the initial boiling point should desirably not exceed 115F. And the maximum gum content of the hydrogenation charge stock ispreferably less than 15 milligrams per 100 milliliters in order tominimize deactivation of the hydrogenation catalyst. It is usuallydesirable to retain the carbon disulfide formed in the pyrolysisreaction in the liquid to be hydrogenated. Carbon disulfide is not atypical constituent or contaminant of petroleum crudes for it is seldompresent therein in quantities greater than a few parts per million.

The liquid feed typically contains about 20 to of aromatic hydrocarbons,mainly as benzene and toluene, when the product is desired as anintermediate suitable for further processing in the production ofaromatic hydrocarbons. Relatively mild hydrogenation conditions are usedin this process because of the high hydrogenation activity of pallidiumcatalysts in general as well as the fact that the presence of thesulfide agent enhances the activity of palladium for hydrogenatinghighly unsaturated hydrocarbons.

In all cases, the maximum temperature in the hydrogenation zone shouldbe kept below the point at which hydrodesulfurization, that is theconversion of organic sulfur compounds to hydrogen sulfide, takesperhaps which is usually at temperatures of 450 F. and higher. It isgenerally preferable to maintain the average hydrogenation temperature(the mean of the inlet and outlet temperatures of the hydrogenationzone) below about 250 F. The average temperature of the exothermichydrogenation reactions may be readily controlled by regulating thetemperature of the materials charged and by adjusting the space velocityin the reactor. Although ambient temperatures are usually preferred forthe charge, this material may be gently heated to temperatures notexceeding about 200 F. under certain substances such as cold weatheroperations or when the activity of the catalyst is decliningsignificantly.

The liquid hourly space velocity is desirably maintained within therange of about 0.2 to 15.0, and preferably between about 0.5 and 8.0. Inthe case of a gaseous charge, the space velocity may be between about0.3 and 25.0 volume of vapor charge per hour per volume of catalyst(VHSV).

The selectivity of a palladium catalyst for catalyzing hydrogen additionto highly unsaturated hydrocarbons, especially conjugated diolefins, inpreference to saturating olefins is substantially enhanced by thepresence of certain sulfides, namely carbon disulfide and hydrogensulfide. Carbon disulfide is greatly preferred as it appears to have aconsiderably greater effect than hydrogen sulfide and moreover iscapable of actually decreasing the undesired saturation of mono-olefinswhile simultaneously increasing the desired hydrogenation of conjugateddiolefins. The reason for this strange increase in selectivity is notknown, and it seems to be specific in nature as hydrogenation reactionsover platinum are not similarly affected and organic sulfur com-poundsin general do not improve the selectivity of a palladium catalyst.

A large excess of hydrogen is charged to reactions of the type describedherein, usually in the form of a hydrogen-rich gas containing Chydrocarbons. When treating a liquid feed, such as pyrolysis gasoline,the hydrogen charge may be within the range of about 500 to 5000standard cubic feet per barrel (s.c.f./b.) of the gasoline and the rangeof about 1200 to 3000 s.c.f./b. is preferred. For hydrogenating gaseoushydrocarbon mixtures, the hydrogen may be charged at an equivalent rateof about 0.5 to 4.8 s.c.f. per s.c.f. of the gaseous hydrocarbon feed.

The total consumption of hydrogen in this process varies of course withthe particular feedstock employed; but in general, it is in the range ofabout -800 s.c.f./b. of liquid feedstock. A typical value is 300s.c.f./b. with a charging stock of diene and bromine numbers of 15 and24, respectively; and the consumption is usually found to be less than500 s.c.f./b. Substantial excesses of hydrogen have been specifiedhereinbefore to avoid a drop in the hydrogenation rates as a result ofan inadequate supply of hydrogen. Although pure hydrogen may be used, itis customarily supplied as a mixture of hydrogen and gaseoushydrocarbons in the otf gases of units for reforming naphthas ofhydrodesulfurizing gas oils, etc. The gas charge prefer-ably has ahydrogen content of at least 60% by volume but gaseous mixtures with aslittle as 40% hydrogen may be used. In describing the charging and theconsumption of hydrogen in s.c.f. or other units, reference is made onlyto the hydrogen content in the case of gaseous mixtures and not to thetotal quantity of a mixed gas which includes a component other thanhydrogen.

The total pressure in the reactor is not critical but the partialpressure of hydrogen at the inlet of the reactor is important inavoiding undesired side reactions, such as the formation of gum or cokeon the'catalysts. The hydrogen partial pressure is desirably maintainedwithin the range of about 200 to 1000 p.s.i. and about 400 to 700 p.s.i.is generally preferred. A major proportion of the product gases withmuch unconsumed hydrogen is preferably recycled to the process, and thisusually constitutes a major proportion of the total quantity of gasescharged to the reactor. Another part of the product may be bled off foruse as a fuel gas, etc., to avoid accumulating excessive amounts ofcontaminants or insert substances in the system.

A palladium catalyst possesses the high hydrogenation activity requiredin order to hydrogenate at relatively low temperatures the more reactiveconjugated diolefins; and this quality is substantially improved by thepresence of carbon disulfide or hydrogen sulfide according to thepresent invention. Its polymerization activity is relatively low so thatthe formation of gums on the catalyst is minimized or eliminated.Furthermore, this catalyst is substantially devoid of alkylationactivity and thus does not promote the undesired alkylation of aromaticswith olefins.

The improvement in a selectivity is obtainable with palladium catalystsin general. However, the catalytic metal is desirably dispersed upon thesurface of various inert, porous carrier materials in particle form suchas tablets or pellets, extruded cylinders or crushed and screenedmaterial of random shape in concentrations of about 0.05 to palladiumbased on the total weight, and the range of about 0.2 to 2.0% ispreferred. Various aluminas are preferred for the purpose, especiallygamma or chi alumina, and a particle size of about to /3 inch isgenerally recommended for fixed bed operations. Catalysts of substantialacid activity are not desirable for this process since they produceunwanted cracking reactions, hence silica-alumina catalyst supports areusually avoided. While it is preferable that the catalyst support besubstantially free of halogens, a relatively low halogen content up toabout 0.5% may be tolerated. Optionally, but desirably, the palladiumcatalyst composite may include a promoter such as chromium oxidesdeposited thereon in an amount, for example, such that the chromiumcontent of the catalyst is about equal to the palladium content. Themanufacture of such catalyst composites is well known in the art andaccordingly need not be further described here.

Regeneration of the catalyst is required. when the diene numberreduction gradually falls oil to an unacceptable level. The inlettemperature may be increased somewhat to delay the need for regenerationbut not to the extent of raising the temperature anywhere in the reactorup to the range where substantial hydrodesulfurization occurs.

Although palladium-omalumina catalysts retain their activity forextremely long periods, as for instance, a year or more, regeneration iseventually necessary; and this may be readily accomplished by heatingthe reactor to a temperature of about 700 to 900 while passing a gascontaining 1 or 2% oxygen through the catalyst bed. A diluent is usuallyintroduced with the air to 6 avoid excessive regeneration temperatureswhich can reduce catalyst activity considerably. Nitrogen, flue gas orthe generally more convenient medium of steam may be utilized as thediluent.

Purging the catalyst with hydrogen at a partial pressure 200 to 500p.s.i. and 750 to 850 F. for between 4 and 16 hours sometimes serves toregenerate it al most as effectively as the aforesaid combustion.Accordingly, it is contemplated that, in the absence of severedeactivation, this catalyst may be regenerated several times by suchtreatment With a hydrogen-rich gas be fore it is necessary to regeneratethe contact agent by the combustion technique.

When treating a pyrolysis gasoline, the relatively mild hydrogenatingconditions described herei-nbefore, and especially the use of relativelylow temperatures no higher than necessary to obtain the desiredhydrogenation of highly unsaturated hydrocarbons, are also supplementedby maintaining a substantial portion of the charge liquid in the liquidphase throughout the hydrogenation reaction. With such liquid feeds, itis preferable to maintain at least 15% and preferably at least 30% byvolume of the normally liquid constituents of the reaction mixture inthe liquid phase. In many operations only a small percent of the liquidcharge is in vapor form during this hydrogen treatment.

Consequently, despite the unstable nature of the hydrocarbon feedstock,very little if any gum is formed in the reactor. The relatively lowreaction temperatures are not conducive to thermal polymerization, andthe catalyst has little or no polymerization activity. Maintaining asubstantial proportion of the reaction mixture in the liquid phaseavoids approaching the point of dryness in the reactor, and therebyfurther lessens the tendency toward polymerization. In addition, theusually substantial aromatic content of this liquid makes it a goodsolvent for polymeric gums, so the liquid phase flowing downwardlythrough this mixed phase reactor dissolves and carries along in solutionmost of any polymer formed therein.

The reaction conditions may be regulated and balanced against oneanother to provide the degree of selective hydrogenation needed to yielda product of the desired qualities. When the diene number reductiontends to become too small to provide the desired stability in theproduct, a more severe or greater degree of hydrogenation of thepyrolysis liquid is required. This may be obtained by decreasing thefeed rate in order to correspondingly reduce the space velocity therebylengthening the time of contact between charge and catalyst. However, inorder to maintain the maximum productive capacity in a commercial plant,instead of reducing the charging rate, it will usually be found moredesirable to gently preheat the charge to a temperature not exceedingabout F. This practice is likely to be particularly beneficial whileoperating under severe winter weather conditions.

If it is desired to diminish the degree of hydrogenation, this isdesirably accomplished by increasing the feed rate. This adjustment, ofcourse, increases. the space velocity (i.e., lowers the residence time)and consequently lowers the reaction temperature somewhat since theexothermic hydrogenation reactions do not proceed as far as before. Alsodiene num'ber reduction may be increased by increasing the concentrationof sulfide agent Within the ranges indicated elsewhere. Both the lowertemperature and the higher production rate are beneficial. While theseverity of hydrogenation can also be decreased by decreasing thepartial pressure of hydrogen at the reactor inlet, for instance, byreducing the total pressure, this is a less practical manner ofadjusting the reaction conditions in commercial practice.

The diene number of the normally liquid fraction of the reactionproducts is generally far less than that of the feedstock, and suchreduction is maintained high enough to inhibit any substantial tendencyfor reactive diolefins to form gums even during prolonged storage. Thisstabilizing of the originally unstable hydrocarbon feed againstsignificant polymerization is better illustrated in terms of asubstantial reduction in diene number (e.g. at least 80% reduction forcertain purposes) which accurately denotes a proportionate decrease inthe content of the most readily polymerizable monomers, rather than inasserting a relatively low diene number as the maximum permissible sincethe latter could include undesirable charges to the reactor in which asmall but significant content of dienes is composed almost entirely ofthe most easily polymerizable diolefins. In the hydrogenation ofmixtures containing both conjugated and unconjugated diolefins, thelatter have reactivities and qualities similar to mono-olefins and thusmay be present in the product in substantial amount.

As indicated earlier, the sulfide agent for enhancing the selectivity ofthe palladium catalyst may be either added at least intermittently tothe charge to the hydrogenation reaction or, in the case of carbondisulfide, it may be formed in the pyrolysis reaction and retained inthe fraction of cracked products, which is selected for the hydrogentreatment. Both of these techniques may be employed in combination inorder to maintain the desired concentration of sulfide agent in thehydrogenation charge. Also, carbon disulfide and hydrogen sulfide may beused together or alternately in the process of this in vention, ingeneral, while, any addition of the sulfide agent is preferably carriedout continuously to maintain a steady concentration thereof,intermittent addition at frequent intervals may also be employed,inasmuch as the effect of the agent seems to last for an appreciableperiod. Accordingly, during the continuous hydrogen treatment, theconcentration of such agent may be expressed as the averageconcentration over a substantial period of such agent in the materialundergoing hydrogenation. The concentration of the sulfide agent shouldbe sufficient to substantially increase the selectivity of the palladiumcatalyst for hydrogenating highly unsaturated hydrocarbons in preferenceto saturating olefins, and suitable amounts may be expressed as thoseproviding at least 100 parts per million (p.p.m.) of sulfur in the formof either carbon disulfide or hydrogen sulfide based on the weight ofthe hydrocarbon charge; the range of about 200 to 500 p.p.m. beingusually preferred. While said average concentration may range up to 1000p.p.m. or more in some cases, it is seldom if ever desirable to allowsuch concentration to reach a level at which the activity of thecatalyst for hydrogenating conjugated diolefins, etc. is substantiallydecreased even if the catalyst selectivity remains the same or isincreased. Usually little or nothing is to to be gained by using saidagents in amounts corresponding to more than about 500 p.p.m. of sulfur.

In the case of hydrogen sulfide, this agent is often available inrefineries, in admixture with hydrogen which is also utilized in theprocess, for example, the unscrubbed off-gas of a catalytichydrodesulfurization unit.

The improvement in selectivity obtained with the instant hydrogenationprocess is realized not only with fresh palladium catalysts but alsowith those which have been partially deactivated in long service. Sincenot only the selectivity but also the activity of a palladium catalystin hydrogenating highly unsaturated hydrocarbons, such as the conjugateddiolefins, is increased at the same average reaction temperature, thespace velocity of the hydrogenation reaction may be increased inmaintaining any previously acceptable level of selective hydrogenationthere-by increasing production. Also the catalyst may be kept on streamlonger in maintaining such a level of hydrogenation before it isnecessary to lower the space velocity to the rate used in the absence ofthe sulfide agent.

After hydrogenating pyrolysis gasoline, the reaction effluent, agaseous-liquid mixture, is cooled and separated in conventionalapparatus and the normally liquid fraction is withdrawn as thestabilized product of the process.

This material may be blended in varying proportions depending on theultimate use with other blending stocks suitable for that use. Forexample, the hydrogenated pyrolysis gasoline may be thoroughly mixedwith one or more gasolines of the alkylate type or those derived fromcatalytic cracking, reforming or coker operations.

In general, it is not necessary to subject the hydrogenated liquidfraction to distillation prior to blending it into other motor fuelcomponents. However, if the gum content is above a desirable level, theproduct liquid may be distilled in order to eliminate the gum in thebottoms while the overhead fraction is utilized in gasoline. If such adistillation is found necessary, it is usually preferable to distill theentire blended gasoline rather than the hydrogenated component thereofbecause of the relatively small amount of nonvolatile matter in thelatter fraction.

For a better understanding of the nature and objects of this invention,reference should be had to the following detailed example.

Example A chromia-prom-oted palladium catalyst on a gammaalumina supportin the form of inch diameter cylindrical particles 7 inch long isemployed as the fixed or stationary bed in a closed reaction vessel forthe hydro genation of pyrolysis gasoline with relatively pure commercialgrade hydrogen p.p.m. carbon monoxide on a molar basis). Based on thetotal weight of the contact mass, there are surface deposits on thealumina of 0.51% of chromium present in oxidized form and 0.50% byweight of palladium metal.

After operating for 11 weeks with pyrolysis gasolines of the same orsimilar characteristics, a 57 API thermally cracked gasoline with aboiling range of to 230 F., a diene number of 2l and a bromine number of48 is charged to the reactor. This gasoline is derived from thermalcracking a mixture of light straight run gasolines and light reformednaphtha originating from a variety of crude oils. The sulfur content ofthis mixture is reduced to a total of about 200 p.p.m. by scrubbing withaqueous caustic soda solution subsequent to the pyrolysis step. Thehydrogenation feedstock contains only about 25 to 35 p.p.m. of carbondisulfide and no significant amount of hydrogen sulfide, for most of thesulfur is present as thiophene. In a single pass continuous operation,this pyrolysis liquid is charged to the reactor at a liquid hourly spacevelocity of 2.0 while the commercial hydrogen is introduced at a rate of1500 s.c.f./ b. of liquid feed. The average reaction temperature is 221F. and the hydrogen partial pressure amounts to about 450 p.s.i.g. in atotal reaction pressure of 500 p.s.i.g. Under these conditions, about40% of the reaction mixture, based on the volume of pyrolysis liquidcharged, is retained in the liqud phase as it flows downward through thecatalyst bed. The reaction products are drawn off at the bottom of thereactor and cooled to about F. before the gaseous and liquid phases areseparated in a conventional separator. Although suitable for thepurpose, the hydrogen-rich product gas is not returned to the reactor inthe instant series of runs. The liquid product is found to have a dienenumber of 5.5 and a bromine number of 26.5 which amount to reductions of74% in the diene number and 45% in the bromine number, respectively.

A second pyrolysis gasoline of generally similar nature and origin isthen substituted for the original feedstock. This particular charge hasa boiling range of 95 to F. and a considerably higher sulfur content of380 p.p.m. resulting from less scrubbing of the pyrolysis material, italso contains approximately 65% by volume of aromatic hydrocarbons,chiefly toluene and benzene. More than three-fourths of this sulfur isin the form of carbon disulfide, that is about 350 to 400 p.p.m. ofcarbon disulfide, with the balance composed essentially of thiopheneplus small amounts of other organic sulfur compounds. Again, nosignificant amount of hydrogen sulfide is present as this substance isseparated beforehand along with the gaseous products of the pyrolysisreaction. When steady state conditions are reached after a substantialperiod in which the average reaction temperature drops to 215, it isfound that the diene number of the normally liquid fraction of thehydrogenation product is 0.5 to 1.0 and its bromine number is 38.5.Thus, the diene number reduction is over 95% and the bromine numberreduction is only 20% in this hydrogen treatment, and this represents asaturation of only about 32% of the olefins formed by hydrogenation ofthe diolefins. Also no appreciable hydrogenation of aromatichydrocarbons takes place. The aforesaid liquid hydrogenation product isan excellent motor fuel blending stock-clean, noncorrosive and having ahigh antiknock rating.

It is evident that the presence of carbon disulfide substantiallyincreases both the activity and selectivity of the palladium catalystfor hydrogenating conjugated diolefins and also decreases the activityof the catalyst in saturating monoolefins even though the palladiumcatalyst is already considered relatively highly selective in thisregard.

Another portion of the second pyrolysis gasoline is charged to a reactorcontaining a platinum catalyst consisting of 0.8% platinum supported ongamma alumina particles for comparison under generally similarconditions except for employing a higher average reaction temperature of235 F. to at least partially compensate for the lower hydrogenationactivity of platinum. However, it is readily apparent that enhancedselectivity is not realized in hydrogenating unsaturated hydrocarbonsover platinum while carbon disulfide is present. Instead the platinumcatalyst is deactivated greatly, for after carrying out this treatmentfor only 24 hours, the bromine number of the liquid product is the sameas that of the charge and the diene number is reduced only 12%.

While the instant method has been described in detail hereinabove, itwill be appreciated by those skilled in the art that the invention iscapable of broad utilization with a great variety of unsaturatedhydrocarbon charging stocks and that the present process is notrestricted to any particular details disclosed other than thosespecifically recited in the appended claims.

I claim:

1. A process for the selective nondestructive hydrogenation ofhydrocarbons which comprises treating an essentially hydrocarbon mixturecontaining a conjugated diolefin and an olefin with hydrogen athydrogenation temperatures below desulfurization temperatures in thepresence both of a catalyst containing palladium and of a sulfide of thegroup consisting of carbon disulfide and hydrogen sulfide wherein saidsulfide substantially increases the selectivity of said catalyst forhydrogenating said diolefin.

2. A process according to claim 1 in which said sulfide is carbondisulfide.

3. A process according to claim 1 in which a substantial proportion ofsaid mixture is maintained in liquid phase.

4. A process according to claim 1 in which the average concentration ofsaid sulfide corresponds to at least 100 parts of sulfur per millionparts by weight of said mixture.

5. A process according to claim 1 in which the average hydrogenationtemperature is below about 250 F.

6. A process according to claim 1 in which said catalyst comprisesbetween about 0.05 and by weight of palladium on a porous carrier.

7. A process according to clai-m 1 in which said catalyst comprises anoxide of chromium and between about 0.2

10 and 2.0% by weight of palladium on a particle form porous carrier.

8. A continuous process according to claim 1 in which said sulfide isadded at least intermittently to said mixture in sufiicient quantity tomaintain an average concentration of at least parts of sulfur in theform of said sulfide per million parts by weight of said mixture.

9. A continuous process according to claim 1 in which carbon disulfideis added at least intermittently to said hydrocarbon mixture insufficient quantity to maintain an average concentration of at leastabout 200 parts of sulfur in the form of carbon disulfide per millionparts by Weight of said hydrocarbon mixture.

10. A process according to claim 1 in which said sulfide is carbondisulfide and a substantial proportion of said mixture is maintained inthe liquid phase.

11. A process according to claim 1 in which thermally cracked petroleumhydrocarbons are employed as said hydrocarbon mixture.

12. A process for the selective nondestructive hydrogenation ofhydrocarbons which comprises treating an essentially hydrocarbon mixturecontaining a conjugated diolefin and an olefin with hydrogen athydrogenation temperatures below desu lfurization temperatures whileretaining a substantial liquid phase in the presence both of a catalystcontaining between about 0.05 and 10% palladium on a porous carrier andof carbon disulfide in an average concentration corresponding to atleast 100 parts of sulfur per million parts by weight of said mixturewherein said carbon disulfide substantially increases the selectivity ofsaid catalyst for hydrogenating said diolefin. 1

13. A process according to claim 12 in which said mixture contains anaverage concentration of carbon disulfide corresponding to between about200 and 500 parts of sulfur, the average hydrogenation temperature isbelow about 250 F. and at least about 15 percent by volume of saidmixture is retained in the liquid phase during said treatment.

14. A process according to claim 12 in which said hydrocarbon mixture isa pyrolysis gasoline.

15. A process for the selective nondestructive hydrogenation ofhydrocarbons which comprises treating a gasoline boiling below about 240F. and containing conjugated diolefins and olefins with hydrogen at anaverage reaction temperature below about 250 F. in the presence of acatalyst comprising an oxide of chromium and between about 0.2 and 2.0%by weight of palladium on a particle form alumina carrier whilemaintaining at least about 30 volume percent of said gasoline in theliquid phase and introducing carbon disulfide at least intermittently insuflicient quantity to maintain an average concentration of betweenabout 200 and 500 parts of sulfur in the form of carbon disulfide permillion parts by weight of said gasoline to substantially increase theselectivity and activity of said catalyst for hydrogenating saiddiolefins and to decrease the activity of said catalyst for saturatingolefins.

References Cited by the Examiner UNITED STATES PATENTS 2,638,438 5/1953Hoffman et al. 208-255 2,914,470 11/1959 Johnson et al. 208-2163,075,024 1/ 1963 Frevel et a l. 260-677 3,084,023 4/ 1963 Anderson etal 260-677 3,098,882 7/1963 Arnold 260-683.9 3,113,983 12/1963 Kirsch etal. 260-677 3,167,498 1/1965 Kronig et al. 208-255 DELBERT E. GANTZ,Primary Examiner. S. P. JONES, Assistant Examiner.

1. A PROCESS FOR THE SELECTIVE NONDESTRUCTIVE HYDROGENATION OFHYDROCARBONS WHICH COMPRISES TREATING AN ESSENTIALLY HYDROCARBON MIXTURECONTAINING A CONJUGATED DIOLEFIN AND AN OLEFIN WITH HYDROGEN AHYDROGENATION TEMPERATURES BELOW DESULFURIZATION TEMPERATURES IN THEPRESENCE BOTH OF A CATALYST CONTAINING PALLADIUM AND OF A SULFIDE OF THEGROUP CONSISTING OF CARBON DISULFIDE AND HYDROGEN SULFIDE WHEREIN SAIDSULFIDE SUBSTANTIALLY INCREASE THE SELECTIVITY OF SAID CATALYST FORHYDROGENATING SAID DIOLEFIN.